CTLA-4 was first described as a negative regulatory molecule by our group in 1994. During the past 11[unreadable] years the molecule has been extensively studied. The molecule has been shown to have an intrinsic effect[unreadable] on T cell activation by directly delivering negative signals to T cells to shut down activation and[unreadable] differentiation. In addition, there have been a number of studies suggesting an extrinsic role for CTLA-4 as[unreadable] the molecule has been proposed to be the effector molecule by which regulatory T cells suppress immunity.[unreadable] All of these "functions" have been complicated by the recent discovery that CTLA-4 can be expressed as a[unreadable] B7 ligand non-binding variant that controls T cell activation. In addition, polymorphisms in the CTLA-4 gene[unreadable] has been linked to Graves Disease and Type 1 Diabetes. Thus, in this renewal, we will continue to addreess[unreadable] the fundamental biology of CTLA-4. We propose to directly address both the intrinsic and extrinsic role of[unreadable] CTLA-4 in the regulation of initial T cell activation, ongoing autoimmune responses and maintenance of[unreadable] tolerance. The following specific aims are proposed: 1. To study the biochemical basis of B7 ligand-dependent[unreadable] and ligand-independent CTLA-4-mediated inhibition of T cell function; 2. To study the intrinsic[unreadable] role of CTLA-4 in T cell development, regulation of tolerance induction and development of diabetes in NOD[unreadable] mice; and 3. To study the intrinsic role of CTLA-4 in T cell development, regulation of tolerance induction and[unreadable] development of diabetes in NOD mice. We will use a combination of novel mice and reagents, combined[unreadable] with TCR transgenic and bone marrow chimera models to identify the cellular and mechanistic basis for[unreadable] CTLA-4 regulation. The results of these studies will provide insights into the mechanisms of CTLA-4[unreadable] regulation of immunity, and test the the role of CTLA-4 in lymphoproliferative/homeostasis versus immune[unreadable] activation and tolerance are mediated by distinct extrinsic versus intrinsic pathways.[unreadable] Importantly, the information learned from this study may have important implications for public health for[unreadable] several reasons. T cells play a central role in immunity and autoimmunity. As such, any means to modulate[unreadable] T cell activity may lead to novel therapeutic approaches to treat these disease. Moreover, the fundamental[unreadable] importance of CTLA-4 in the induction and maintenance of peripheral tolerance is central to our[unreadable] understanding of disease etiology and current therapeutic approaches.